CFD Modeling of Activated Sludge Mixing in an Oxidation Ditch Conversion

Abstract

The treatment process at the City of Woodland's Water Pollution Control Facility (WPCF) is being enhanced to improve nitrogen removal. The existing oxidation ditches will be converted to a once through treatment process. Computational fluid dynamics (CFD) modeling was used to evaluate mixing of solids with the converted tank, using a solids transport equation within the computation. The type, location, and orientation of mixers in anoxic regions were evaluated to ensure solids remained in suspension and well mixed. The aerated zone mixing was evaluated with a multi-phase model to verify the injected air would mix the complete trapezoidal cross section, including through a 180-degree bend.

Modeling, Instrumentation, Automation, and Optimization of Water Resource Recovery Facilities

“…Suzuki et al (2013) used a modified ADM1 model enhanced to accounted for mechanisms specific to THP high solids digestion and determined that the VFA level was very sensitive to the solids concentration influent to the digesters downstream of THP, since VFAs increased from 1,000 to 3,000 mg/L for influent solids increasing from 12 to 14%. Burger and Parker (2013) During the design analysis of 1:1 side-slope oxidation ditch to be converted to an MLE process, Wicklein et al (2013) used 3D CFD modeling to locate, size and orient anoxic zone mechanical mixers and to confirm that the diffused air mixing pattern was sufficient to mix the full aerobic volume through a rolling mixing pattern. It has been widely demonstrated that the filtration performances in MBRs can be improved with a two-phase flow (sludge-air) or higher…”